1. Field of the Invention
The present invention relates to a curved surface forming apparatus that is capable of forming a curved surface and adjusting and holding the formed curved surface, an optical scanning apparatus to which the curved surface forming apparatus is applied, and an image forming apparatus that includes the optical scanning apparatus.
2. Description of Related Art
In recent years, there have been developed various techniques for an optical writing apparatus and image forming apparatus that are capable of reducing curvature of a scanning line formed by a light beam.
For example, Japanese Patent No. 3324302 discloses an image forming apparatus. This image forming apparatus includes a reflecting mirror that forms part of a light beam scanning optical system, and receives and reflects a light beam by a part of a first surface between a first position and a second position different from the first position; a mirror supporting mechanism that is in contact with the first surface receiving the light beam and supports the first surface at two positions having a scanning area, which connects the first position and the second position of the first surface, interposed therebetween with a predetermined distance; a first pressing mechanism that presses both end portions on a second surface of the reflecting mirror, the second surface being not supported by the mirror supporting mechanism; a second pressing mechanism that presses the second surface at a position closer to center from a portion of the reflecting mirror supported by the mirror supporting mechanism; and a pressing force adjusting mechanism that adjusts a pressing force produced by the second pressing mechanism according to curvature of an optical system. The pressing force adjusting mechanism adjusts curvature of a scanning line formed by the optical beam by allowing adjustment of the pressing force that deflects the reflecting mirror in an optical scanning path. The pressing force adjusting mechanism, which adjusts the pressing force that deflects the reflecting mirror, has an adjusting screw, an adjusting plate to which the adjusting screw is attached, and a fixing member that fixes the adjusting plate.
However, in the case of the image forming apparatus disclosed in Japanese Patent No. 3324302, a large number of parts and assembling steps required to produce the pressing force adjusting mechanism lead to the complex structure. This results in difficulty in responding to the request that the mechanism can adjust the curvature of the scanning line in a narrow region, and poor flexibility in light-ray bending arrangement in the optical scanning apparatus. Thus, there are problems that it is difficult to achieve miniaturization of the entire image forming apparatus, and that such complex structure of the pressing force adjusting mechanism causes an increase in manufacturing cost.
Meanwhile, for example, Japanese Patent Application Publication No. 2006-017881 discloses an optical writing apparatus having an optical housing in which multiple optical members are arranged to irradiate corresponding photoreceptors with scanning lines formed by light beams emitted from multiple light sources. This optical writing apparatus includes a holding member that holds a reflecting mirror, and a scanning line curvature adjusting mechanism. Here, the reflecting mirror is one of the optical members and is provided on an optical path of each light beam. The scanning line curvature adjusting mechanism is provided at a substantially central portion in a longitudinal direction of the reflecting mirror and adjusts the amount of deflection of the reflecting mirror in a direction in which the reflecting mirror curves in a concave or convex shape with respect to an incident direction of the light beam entering the reflecting mirror.
However, in the optical writing apparatus disclosed in Japanese Patent Application Publication No. 2006-017881, the scanning line curvature adjusting mechanism has a structure that adjusts fluctuations in the amount of curvature of the reflecting mirror after setting the reflecting mirror to have any one of a concave curvature and a convex curvature with application of an initial pressing force. As a result, there is difficulty in providing a wide range of adjusting area crossing over curvature 0 (infinity R) from concave to convex or conversely from convex to concave. Therefore, even if the curvature of a scanning line is minute, adjustment work must be carried out as in the conventional optical system, and this causes problems of increasing the number of adjusting steps and its manufacturing cost.